I have used a mouse ball and valve grinding compound. I open the chuck wide open on my 18 volt electric drill and gently start the ball turning with a slight down pressure on top of the ball. The chuck teeth apply just the right amount of grip or tension to spin the ball.I make sure the drill is vertical and strait up over the bore. In your case cutting a square cut would be critical to give you a flat face to work off of.The mouse ball and or a large round head brass screw chucked tight will give a good crown if you take your time. I had a friend crown an old 22 remington tube feed my Dad bought during the depression for $3.50.So some rifles are irreplaceable. Oh and the accuracy returned after a good cleaning and crowning.
